In practical battery management applications, the battery is disconnected from the power path when the battery voltage drops below a threshold, so as to prevent the battery from being drained too much. However, this over discharge protection method will cause some issues under the condition of large discharge current.
As shown in FIG. 1, the battery is illustrated as a block having a capacitor and an internal resistor, the battery discharge circuit is coupled between the battery and a load and used to provide the energy stored in the battery to the load. FIG. 2 illustrates waveforms of the battery discharge circuit shown in FIG. 1 under large discharge current. As shown in FIG. 2, when the discharge current Idchg is large, the voltage across the internal resistor is also large. The battery voltage Vbat is quickly reduced to reach the threshold voltage Vth and the over discharge protection is triggered. The battery and the battery discharge circuit are disconnected, the voltage across the internal resistor as well as the discharge current Idchg is changed into zero. The battery voltage Vbat increases because of the battery internal resistor effect and relaxation effect. This means, there is still some battery capacity remained and not used.